First of all, why would you want the wii on the dmz? There a no online games yet on wii, so nothing is being hosted. All a DMZ is forward all incoming requests to that one device. I doubt anyone is trying to request info from your wii. I see you have an airport? Why you running 2 routers? The airport can be the only router on the network. The problem i see you having now is that the airport wireless network is piggy backing on the main main network. You essentially have 2 networks. On wired, one wireless that cant talk to each other. One way to bypass that is make it all 1 network. If your having the airport assign its own IP addresses to devices, you can set the ip adresses it gives out in the same net work as the other router. Just make sure both don't assign the same addresses. Basically have the main router assign 192.168.1. 3-99 and the airport assign 192.168.1. 100-200. Set the main router as 192.168.1.1 and the airport as 192.168.1.2. Make sure the subnet mask on both is 255.255.255.0. Now all devices will be on the same network and you can assign a DMZ through the main router that will be forwarded through the airport since they are all the same network now.
Ok having to DHCP servers will not combine the networks you need to disable NAT. In the Internet tab of the airport set it to DHCP (receives its own address by dhcp) then in network you want to turn off Distribute IP address. This will then combine the networks without having two DHCP servers. Also if you just changed the IPs it serves without turning off NAT the same problem would occur. So just disable distributing IPs all together this disables NAT. (atleast on the old extremes) Then you have a single network. After this is done go to your other router and set up the DMZ or port forward the ports you need)
If the Router supports MAC-Address-Level settings then you can just use the Wii's MAC address to get the Static IP instead of disabling DHCP completely which affects every computer on the network
For your router though..
192.168.0.1 -> Enter Username/Password
Click "NAT Setup"
Click "DMZ Host Setup"
And go from there - i dont have this router, so i could only get that far from a few images i found online (From www.portforward.com ) - it shouldnt be that complicated though